Telecommunications Service Providers (TSPs) are discovering that new technologies may offer opportunities for better service at lower cost. As part of their core business, TSPs provide electronic communication paths between customers. In their original form, these paths were dedicated telegraph wires between customers. After a short time, switching centers were invented and these paths took the form of wires between the customers and the local switching centers. The operators at the local switching centers were able to manually relay messages from one wire to another and were also able to electrically connect any of the wires so that local customers could communicate directly.
With the advent of the telephone, the single wires were replaced with closed circuit “loops” between the customers and the switching centers and the customer base began increasing rapidly. Before long, the local switching centers were connected via multiconductor “trunk lines” and long distance communications became possible between customers of different switching centers. Initially, for both local and long distance, the connecting of different circuits was performed manually by operators. As electronics technology improved, the connecting was done using arrays of electronic relays that switched in response to electric pulses sent by a customer's phone as the customer dialed. Until this time, all communication paths were established using what is now known as “circuit switching”. In other words, a communication path (i.e., a closed electrical circuit between the customers) was established that remained fixed until the communication ended.
With the evolution of transistors, the TSPs developed ways to transport multiple conversations on the same circuit. One method involved frequency division multiplexing (FDM), in which analog processing was done on each end of a trunk line to shift a communication from the normal frequency band (“baseband”) to one of several higher frequency slots for transport across the trunk line and to shift the communication back to baseband at the other end. As technology continued to evolve, TSPs began performing analog-to-digital conversion to send the communication across the trunk lines digitally. Digital communication allowed for the use of FDM but also allowed for the use of time division multiplexing (TDM) in which different communications are intermixed in time in a predetermined way and reconstructed at the other end.
The analog and digital processing for FDM and TDM typically replaced a physical circuit as a communication path with allocated slots in frequency and time on a given conductor. In operation, these slots are for most purposes equivalent to physical circuits and hence they may be referred to as “virtual circuits”. The term “circuit switching” still applies because the communication path remains fixed until the communication ends. Some TSPs are replacing or augmenting trunk lines with trunks of optical fibers. Digital TDM technology has been applied to these optical trunks as well.
The recent growth in data communications presents a challenge to the communications capacity of TSP networks that were primarily designed to carry telephone conversations. To minimize unused bandwidth (e.g., unused frequency and time domain slots), another technology has been developed. Referred to as asynchronous transfer mode (ATM), this technology has been developed as an industry standard that facilitates the transport of mixed communication types such as voice, video, and data. It relies on computerized switches to perform “packet switching”. Unlike circuit switching, packet switching allows for the use of different circuits during the communication and may even allow for out-of-order delivery of portions of the communication. A customer communication is digitized, converted into data packets, and routed towards the destination via any available bandwidth. At the other end, the packets are sorted and reassembled to reconstruct the customer communication.
TSPs wishing to switch from one technology to another are invariably faced with a changeover period in which both old and new technologies co-exist. In the case of the changeover to ATM, a TSP's network is likely to have trunks using the older TDM technology contemporaneously with trunks using the newer ATM technology. Systems designed for managing the older technology may not be suitable for managing the newer technology and, conversely, adding short-term functionality to newer systems may be inefficient in view of already existing systems for managing the older technology.
When a transition from one technology to another is in progress, equipment using the newer packet-based technologies such as ATM can be referred to as migrated technologies. Equipment still using the older circuit-based technologies such as FDM and TDM can be referred to as non-migrated technologies.